Susan (Anna Maxwell Martin), Millie (Rachael Stirling), Lucy (Sophie Rundle), and Jean (Julie Graham) worked together at top-security HQ Bletchley Park to help crack German military codes, producing crucial intelligence that shortened the war. Now, in 1952, they have returned to postwar domesticity. Although Susan appears to be a typical mother and wife, when she hears about a string of unsolved murders in London, her old Bletchley spirit is ignited and she reconvenes the Bletchley foursome to figure out the pattern behind a serial killer's rampage, in our new favorite PBS series.